I have a Nixie tube clock based on IN-12. I assembled the kit, it works great and it was great fun. But the tubes are just too bright in dark conditions.

I would like to modify the clock to be able to change the brightness. I have included the full schematic bellow.

Nixie clock schematic

I feel that a well placed potentiometer on the high-voltage power-supply (lower right of the schematic) could do the job.


I change the voltage, as expected, I got some non-uniform lighting. reprogramming the microcontroller is more work than I was willing to put in this project.

  • \$\begingroup\$ Place a filter in front \$\endgroup\$ – JIm Dearden May 12 '16 at 12:56
  • \$\begingroup\$ @JImDearden , what do you mean by a filter? And where would you put it? Do you mean putting something semi-transparent in front of the tubes, that would defeat the purpose of the clock. \$\endgroup\$ – RYegavian May 12 '16 at 13:00
  • \$\begingroup\$ There are all sorts of optical filters, some photo reactive . The simplest would be a neutral density filter that would reduce light intensity but not colour. You do not specify in the question anything about the purpose of the clock, merely you wanted to change the brightness. \$\endgroup\$ – JIm Dearden May 12 '16 at 13:11
  • \$\begingroup\$ You are right, an optical filter would indeed reduce the brightness and it would be very simple. But the purpose of the clock is its look, I would rather not put something in front of it. \$\endgroup\$ – RYegavian May 12 '16 at 14:12
  • \$\begingroup\$ Dive-color the individual tubes with red lamp color. That's what the manufacturers of such displays did in the past. \$\endgroup\$ – Janka Feb 8 '17 at 13:32

Don't mess with the voltage, that will cause uneven lighting.

Generally for glow discharge tubes like nixies you can get much better control by PWM than by messing with voltage, so I would suggest programming the tiny to PWM the A1-A4 pins. You will want to make the frequency high enough to avoid quench effects (still a thing even in glow discharge regime) and audio frequencies (Actually it looks like 100-500Hz also works because the time constant is really long).

An explanation is given at Wendt's page. He used a 555 to generate the PWM, but the tiny probably can just generate it internally.

As a bonus, you can change the brightnesses on a per tube basis.


I think the simplest way would be to alter the on time of the segments by changing the microcontroller code. By maintaining the same period for the scan of each digit but turning the digit 'off' for part of the time you could change the brightness without any hardware modifications. Clean and simple, but you need to be able to tell the chip what brightness you want (spare ADC pin?) and of course it may not be easy to change someone else's code.

Trying to PWM with a 555 as Paul suggests should work if you can get it to work with the frequency high enough, but if not it would tend to beat with the scan frequency to cause distracting ripples of brightness in the display.

Your suggestion of a series pot in the HV has a chance of working- the individual neon tubes may not track well enough, and you should take care to avoid contact with the high voltage. I think I'd give that a try first. Try 50K or 100K. Just try some resistors if you don't have a suitable pot on hand. Basically cut the connection at "HV", leaving all the power supply parts on one side and the neon drivers on the other side and add the variable resistance.

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    \$\begingroup\$ Good call on the beating issue. You should be able to just match the scan and dimming frequencies, since both would be coming from the microcontroller clock. Another reason to avoid the 555. \$\endgroup\$ – Paul Foster May 12 '16 at 15:05
  • \$\begingroup\$ Changing the voltage (using P2) didn't do the trick. I'm looking into ways to change the tiny code, using a raspberry pi. It seems rather involved. I'm considering buying this product which provide dimming. \$\endgroup\$ – RYegavian May 13 '16 at 8:54
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    \$\begingroup\$ You didn't try the series resistor then.. \$\endgroup\$ – Spehro Pefhany May 13 '16 at 11:42
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    \$\begingroup\$ I predict that the behavior will be quite different. Also P2 does not have much adjustment range. \$\endgroup\$ – Spehro Pefhany May 13 '16 at 13:06
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    \$\begingroup\$ It wouldn't be for me (I can always jumper back over it with a bit of bare wire and solder). It shouldn't be for most folks with moderate skills. \$\endgroup\$ – Spehro Pefhany May 15 '16 at 21:26

This simplest way to reduce the brightness given the existing circuit is to reduce the high voltage. The brightness will be non-linear with that voltage, but you should be able to find a point that has the brightness you want without it being too dependent on small changes in the high voltage.

The high voltage is generated by a boost converter run by a 666 555 timer:

It looks like when the voltage gets high enough, T12 is turned on, which kills the oscillations. That's pretty crude, will have a strong temperature dependency, and will be touchy to adjust. However, P2 is already there for the purpose of adjusting the high voltage. The more you turn the wiper to the R4 end, the lower the high voltage.

If the existing adjustment range doesn't allow the brightness to get low enough, make R3 a little bigger and/or R4 a little smaller.

If the adjustment is too finicky, put a zener diode in series with the base of T12 and add maybe a 1 MΩ resistor to ground right on the base of T12. That will change the setting range of the high voltage significantly, so R3 and R4 need to be adjusted accordingly. Make sure that the maximum voltage the tubes, D5, and T1 can handle can't ever be attained no matter how P2 is set.

  • \$\begingroup\$ As it was the simplest things to do, I tried first to adjust P2. As suggested by Spehro Pefhany and Paul Foster, the brightness did not change much but it did mess with the lighting uniformity. \$\endgroup\$ – RYegavian May 13 '16 at 8:41

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